Electroconductive silicon nitride based composite sintered body and method for preparation thereof

ABSTRACT

A conductive silicon nitride composite sintered body having an average grain size of 100 nm or less and whose relative roughness (Ra) after electric discharge machining is 0.3 μm or less can be obtained by grinding/mixing a silicon nitride powder and a metal powder together until the average particle size of the silicon nitride powder becomes 30 nm or less, and subsequently by molding and sintering. It is preferable that the crushing/mixing is continued until it is apparent that a peak of added metal in an X-ray diffraction pattern has disappeared during the crushing/mixing.

TECHNICAL FIELD

The present invention relates to a silicon nitride composite sinteredbody having electric conductivity and which is useful as variousstructural parts and wear resistance parts such as cutting tools andsliding parts.

BACKGROUND ART

Silicon nitride has been widely used for various structural parts suchas vehicle engine parts, cutting tools, and abrasion resistancematerials such as roller bearings, and the like, since it is excellentin hardness, mechanical strength, and thermal resistance, and alsochemically stable. Recently, in all of these fields, not only have thelevels of performance required of materials become extremely high butalso the degrees of processing accuracy required of those materials havebecome strict. As a result, when these materials are used for products,the costs of the products become higher as the costs of processing arehigher, which has been the greatest factor in preventing expansion ofthe markets.

Thus, various processing methods have been proposed. Among them, themost frequently used method is a technique of electric dischargemachining that is performed in a state wherein electric conductivity isafforded to a silicon nitride composite sintered body by dispersingconductive particles into a matrix composed of silicon nitride and grainboundary phase.

For example, in Ceramics 21: pp 719–725 (1986), it is described thatelectric discharge machining is made possible by dispersing conductiveparticles of from 20 to 40% by volume into Si₃N₄ to make a siliconnitride material with electric conductivity. However, the surfaceroughness of such silicon nitride materials deteriorates after theelectric discharge machining, and surface cracks occur which areattributed to thermal shock and electric discharge at the electricdischarge machining. Thus, the resultant materials cannot be used forpractical application because of their low mechanical properties unlessgrinding or polishing is carried out to remove their surface cracksafter the electric discharge machining.

DISCLOSURE OF INVENTION

The present invention was derived to solve the above problems. That is,the present invention provides a conductive silicon nitride compositesintered body capable of surface smoothing after the electric dischargemachining, which is excellent in mechanical properties, and a processfor the production thereof.

The conductive silicon nitride composite sintered body of the presentinvention, which is composed mainly of silicon nitride and metal nitrideeach having an average grain size of 100 nm or less, wherein the metalnitride is contained at 25% to 60% by volume, is characterized in thatthere exists a region where the area ratio of the metal nitride iswithin a 2/3 power ratio of volume % of the metal nitride±10% at aregion of 2 μm wide and 2 μm long in an arbitrarily selected section ofthe sintered body. The area ratio herein is a percentage of an areaexcluding the vacancies. The surface roughness (Ra) of the dischargedsurface is 0.3 μm or less after the electric discharge machining. Themetal of the metal nitride is at least one kind of element selected fromGroups IVa, Va and VIa, and more preferably is Ti or Ta.

The sintered body of the present invention is obtained by amanufacturing process comprising a step of preparing a silicon nitridepowder, a powder of sintering aids, and a metal powder, a step ofgrinding/mixing these powders until the silicon nitride powder of whichthe average particle size is 30 nm or less is obtained to make a mixedpowder, a step of molding the mixed powder to make a molded body, and astep of sintering the molded body in a non-oxidative atmosphere to makea sintered body. It is desirable to grind and mix until an X-raydiffraction peak of the metal in the powder apparently disappears in thestep of grinding/mixing. The metal is at least one kind of element fromGroups IVa, Va and VIa, and is preferably Ti or Ta.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration showing a sectional contexture of thesilicon nitride composite sintered body of the present invention whereinthe area ratio of TiN is within a 2/3 power ratio of volume % of TiN±10%at a region of 2 μm wide and 2 μm long (white portion 1 is Si₃N₄, andblack portion is TiN).

FIG. 2 is a schematic illustration showing a sectional contexture of thesilicon nitride composite sintered body wherein the area ratio of TiN isnot within a 2/3 power ratio of volume % of TiN±10% at a region of 2 μmwide and 2 μm long and within a 2/3 power ratio of volume % of TiN±10%at a whole region (white portion 1 is Si₃N₄, and black portion 2 isTiN).

BEST MODE FOR CARRYING OUT THE INVENTION

The composite sintered body of the present invention is described indetail below including the process for the production thereof. “Siliconnitride” in the present invention indicates silicon nitride (Si₃N₄)and/or ceramics containing sialon as a major crystal phase and includesoxynitriding silicon and the like. Also, “metal nitride” includes thosemixed in with oxygen. Further, “silicon nitride composite sintered body”indicates materials wherein a different component is dispersed andcompounded in the matrix composed mainly of such ceramics as the majorcrystal phase.

Each average grain size of the silicon nitride particles and thedispersed particles composed mainly of the metal nitride in thecomposite sintered body of the present invention is 100 nm or less. Inthe case of utilizing the composite sintered body with a larger averagegrain size than this, the relative surface roughness becomes worse afterthe electric discharge machining because the materials are noteffectively eliminated at the electric discharge machining.

Electric conductivity is required to enable electric dischargemachining. Therefore, it is necessary to disperse conductive particlesin a material uniformly so as to be contained therein in network form onthe order of a nanometer. The preferable added amount of the metalnitride which is in the form of dispersed particles is 25% by volume ormore and 60% by volume or less. When it is less than 25% by volume, themetal nitride network is not dispersed uniformly, so it is difficult toobtain the electric conductivity required for the electric dischargemachining. On the other hand, when it is more than 60% by volume,particle growth readily occurs because the dispersed particles tend tocombine with one another during sintering, which results indeterioration of the mechanical properties.

There exists a region where an area ratio of the metal nitride is withina 2/3 power ratio of volume % of the metal nitride±10% at a region of 2μm wide and 2 μm long in an arbitrarily selected section of the sinteredbody. The relative surface roughness deteriorates after the electricdischarge machining because the contexture of such materials of thesintered body that are not within the range are ununiform.

Electric discharge machining becomes possible from 5% to 60% by volumeof the conductive dispersed particles by dispersing the metal nitridesuch that there exists a region where the area ratio of the metalnitride is within a 2/3 power ratio of volume % of the metal nitride±10%at a region of 10 μm wide and 10 μm long in an arbitrarily selectedsection of the sintered body and where the area ratio of the metalnitride is not within a 2/3 power ratio of volume % of the metalnitride±10% at an arbitrarily selected region of 2 μm wide and 2 μmlong. A material dispersing the metal nitride uniformly within the rangehas a surface relative roughness (Ra) of 0.3 μm or less at thedischarged surface after the electric discharge machining. The siliconnitride composite sintered body that has a discharged surface whoserelative roughness (Ra) is 0.3 μm or less is a material having a smoothdischarged surface and excellent mechanical properties after theelectric discharge machining.

The nitride that is to be dispersed particles may be the nitride of atransition metal with electric conductivity. However, in order tosufficiently achieve the target effect of the present invention,preferable is the nitride of an element from Group IVa (Ti, Zr, or Hf),Group Va (V, Nb, or Ta) and Group VIa (Cr, Mo, or W) of the PeriodicTable. Among them, particularly the nitride of Ti or Ta is morepreferable.

Some amounts of carbon and oxygen may be contained in the metal nitridein the form of dispersed particles. For example, the form in whichnitrogen is partially compounded with carbon and/or oxygen is allowed,and carbon and/or oxygen may exist alone. Also, the nitrides containingmultiple metal elements (composite nitride) may co-exist. In order tomake the multiple nitrides of metal elements thus co-exist, powders ofmultiple metals are previously mixed. Or they may be added in a form ofcompound of metals or a composite nitride. Added particles of differentmetal elements are dispersed from one another by adding in a formcontaining two or more kinds of metal elements in this way. Sinceparticle growth is inhibited by one another as a result of suchdispersion, the dispersed average particle size of the dispersedparticles becomes smaller than that in the case of containing one type.This results in further improvement of the relative surface roughness ofthe discharged surface.

The material of the present invention is obtained by the processcomprising a step of preparing a silicon nitride powder, a metal powderand a powder of sintering aids, a step of grinding/mixing these powdersuntil the average particle size of the silicon nitride powder becomes 30nm or less so as to make a mixed powder, a step of molding the mixedpowder to make a molded body, and a step of sintering the molded body ina non-oxidative atmosphere to make a sintered body. The sintering in thenon-oxidative atmosphere is preferably performed at a temperature rangefrom 1100° C. to 1500° C. It is preferable that the crushing/mixing iscontinued until it is apparent that a peak of added metal in an X-raydiffraction pattern has disappeared during crushing/mixing.

The raw material powders may be any of those commercially available. Thecrystal type of Si₃N₄ may be either α or β type. And either imidedecomposed powder or directly nitrided powder may be used. Both Si₃N₄powder and powder of the sintering aids preferably have an averageparticle size of 5 μm or less, and more preferably 2 μm or less, sincethe smaller the average particle size, the more desirable in terms ofease of particle size control and improvement in the mechanicalproperties. The smaller the average particle size of the metal powderadded for the dispersed particles, the more desirable it is, and it ispreferably about 10 μm or less and more preferably 5 μm or less.

Grinding and mixing are carried out such that the average particle sizeof silicon nitride powder in the grinded/mixed mix powder is 30 nm orless. If the average particle size of silicon nitride powder is morethan 30 nm, it is difficult to control the average particle size of thesilicon nitride particles and the metal nitride particles to be 100 nmor less and the contexture of the sintered body become uniform. As aresult, the relative surface roughness deteriorates after the electricdischarge machining. It is desirable that mixing is carried out by amethod with grinding means such as a ball mill or attritor. For example,as described in Japanese Laid-Open Patent No. 338576/1998, mechanicalalloying is carried out using such a mixing apparatus. According to thismeans, a fine mixed powder with an average particle size of 30 nm orless is obtained by plastic deformation ability of the metal powder thatis added as the source of particles to be dispersed as mentioned above.Conditions such as grinding acceleration, charging amount ratio betweena powder and a powder grinding medium, grinding time period and the likeare appropriately selected according to the level of the averageparticle size of the initial raw material powder.

A Si₃N₄ powder, a powder of sintering aids and a metal powder to becomedispersed particles may be extremely fine particles obtained beforehandby chemical or physical means from metals and organic/inorganic salts.Such means include, for example, a method in which co-precipitates areobtained from organic salts of metals, a method in which particles areproduced by heat treatment from an inorganic composite compound such asSi—Ti—N, and the like. Also, the grinding means includes a vibrationgrinding method and the like in addition to the above.

We investigated in detail the relationship between the X-ray diffractionpattern of the mixed powder during the crushing/mixing. As a result, anX-ray diffraction peak of added metal in the X-ray diffraction patternof the mixed powder deteriorates during the mixing and apparentlydisappears after a certain duration of mixing. We discovered that thecontexture of a sintered body of the mixed powder of which an X-raydiffraction peak of added metal disappeared apparently showed a regionwhere the area ratio of the metal nitride was within a 2/3 power ratioof volume % of the metal nitride±10% at a region of 2 μm wide and 2 μmlong in an arbitrarily selected section and the metal nitride wasdispersed evenly. It is considered that the silicon nitride powder isdispersed in the metal during mixing and the metal is converted tonitride. A fine structure of the silicon nitride and even dispersion ofthe metal nitride is then obtained.

The molding methods known in the art such as a common dry press moldingmethod, an extrusion molding method, a doctor blade molding method, andan injection molding method can be used for the mixed powder prepared inthe above manner. The most preferable molding method can be selected forquality and production in accordance with the desired shape. The bulkdensity can be also previously increased to enhance moldability bygranulating a mixed powder into granules prior to the molding after thegrinding and mixing.

It is preferable that the molded body is sintered at a temperature rangefrom 1100° C. to 1500° C. in a non-oxidative atmosphere. With such atemperature range the relative density of the sintered body becomes 95%or more, which results in the stable mechanical strength thereof beingreadily obtained after electric discharge machining. The heating meansof sintering may be a common sintering at normal pressure, but arepreferably the means such as a pulse electric discharge sintering methodand a sintering method by high-frequency induction heating in which thetemperature of the molded body can be raised in a short time and themolded body can be uniformly heated. The sintering may be carried outunder a pressure applied with an ambient gas or applied mechanicallyfrom outside. When the sintering temperature is 1100° C. or less, themolded body is not sintered sufficiently. When it is more than 1500° C.,grain growth becomes remarkable and thus it becomes difficult to obtainthe composite sintered body of the present invention.

EXAMPLE 1

An α type silicon nitride powder with an average particle size of 0.8 μmand a metal Ti powder with an average particle size of 10 μm, as well assintering aids Y₂O₃ of 2.5% by weight and Al₂O₃ of 1% by weight,respectively relative to the weight of silicon nitride were prepared.All of the powders are commercially available. The amount of Ti shown inTable I was prepared, and subsequently, mixing was carried out for thetime period shown in Table I using a planetary ball mill having anacceleration of 150 G. The amount of the metal Ti to be added herein isan amount calculated on the assumption that the whole Ti would beconverted into metal nitride. The presence or absence of the metallicpeak was determined by X-ray diffraction for the resultant powder. Theaverage particle size was calculated from the X-ray diffraction data ofthe sample using Hall's formula described in Waseda and Matsubara,“X-ray Structural analysis” page 119–126 (Apr. 10, 1998, published byUchida Rohkakuho). The results are shown in Table I.

TABLE I Ti added Amount Presence or Particle (Amount Mixing time absenceof size of converted as TiN) period (Hr) metallic Ti peak Si₃N₄ (nm)1-1* 25 2 Yes 100 1-2* 25 8 Yes 30 1-3 25 16 No 25 1-4 25 24 No 16 1-5*40 8 Yes 50 1-6 40 24 No 12 1-7* 50 8 Yes 30 1-8 50 24 No 10 1-9 60 16No 30 1-10* 70 24 No 12 1-11* 20 24 No 20 Asterisk (*) indicatesComparative Example

The produced mixed powder was pressurized and sintered by a pulseelectric heating method with a pressure of 30 MPa in a nitrogenatmosphere under conditions shown in Table II. Specular surfaceprocessing was provided onto the surface of the resultant sintered body.Subsequently, the image of the polished surface of the sample was takenby electron microscopy at a magnification of 50,000. The average grainsizes of Si₃N₄ particles and the dispersed particles (TiN) wereconfirmed by statistically analyzing the image in a given visual fieldusing the segment division method.

TABLE II Sintering Relative Grain size Grain size Relative roughnessFracture temperature density of Si₈N₄ of TiN Electric of dischargedsurface strength Area ratio (° C.) (%) (nm) (nm) conductivity (μm) (MPa)of TiN 1-1* 1300 98 300 1000 ∘ 2.0 200 x 1-2* 1300 99 80 600 ∘ 1.5 320 x1-3 1100 98 60 50 ∘ 0.3 900 ∘ 1-3 1300 99 65 70 ∘ 0.2 1050 ∘ 1-4 1100 9860 50 ∘ 0.15 1200 ∘ 1-4 1500 99 80 80 ∘ 0.2 1060 ∘ 1-4* 1600 99.5 120150 ∘ 0.5 540 ∘ 1-5* 1100 98 60 50 ∘ 0.6 600 x 1-6 1100 98 60 50 ∘ 0.151250 ∘ 1-7* 1100 98 60 70 ∘ 0.8 360 x 1-8 1100 99 50 50 ∘ 0.2 850 ∘ 1-91100 99 50 60 ∘ 0.13 800 ∘ 1-10* 1100 99 50 70 ∘ 0.1 330 ∘ 1-11* 130098.5 60 60 x Impossible to — ∘ machine Asterisk(*) indicates ComparativeExamples

Further, the results of the properties examined as described below areshown in Table II. The electric conductivity was represented by a circlein a case where the electric resistance capable of performing theelectric discharge is 10⁰ Ω·cm or less when measured by a four-pointelectric resistance measuring instrument, and by a cross when it is morethan 10⁰ Ω·cm. Further, the samples were cut using an electric dischargemachine (Sodick AP450), and the relative roughness (Ra) of thedischarged surface was determined by a contact type surface roughnessmeter. Also, the samples were finished to the shape of a strength testpiece defined in JIS R1601, and the three-point flexural strength wasdetermined according to the definition of the JIS standard. The surfacethat was subject to tensile stress during the strength test had beenleft in the same state as it was subjected to electric dischargedmachining. The images of the polished surfaces of the samples were takenby a 5,000 magnification electron microscope. Subsequently, an arearatio of TiN particles was calculated at a region of 2 μm wide and 2 μmlong, which were taken arbitrarily. The samples in which the area ratiois within a 2/3 power ratio of volume % of TiN±10% were represented bycircles, and those otherwise were represented by crosses.

FIG. 1 shows a schematic illustration of a sectional contexture of asample in which the area ratio of TiN particles is within a 2/3 powerratio of volume % of TiN±10% at a region of 2 μm wide and 2 μm long. Inthe figure, a white part 1 and a black part 2 indicate Si₃N₄ and TiN,respectively. FIG. 2 shows a schematic illustration of a sectionalcontexture of a sample in which the area ratio of TiN particles is notwithin a 2/3 power ratio of volume % of TiN±10% at a region of 2 μm wideand 2 μm long. It is shown that the area ratio of TiN particles iswithin a 2/3 power ratio of volume % of TiN±10% in the entire FIG. 2,whereas it is not within a 2/3 power ratio of volume % of TiN±10% at anarbitrarily selected region of 2 μm wide and 2 μm long as shown in thefigure.

As can be seen in Tables I and II, the sintered body of which theaverage grain size is at most 100 nm was obtained when crushing/mixingis continued until a peak of metal Ti in an X-ray diffraction patterndisappears. The relative density in the sintered body of the presentinvention is 95% or more, and the metal nitride (TiN) is dispersedunevenly. Also, the relative roughness (Ra) of the discharged surfaceafter electric discharge machining is 0.3 μm or less, and the fracturestrength is as high as 800 MPa or more.

INDUSTRIAL APPLICABILITY

According to the present invention, electric discharge machining is madepossible by dispersing fine particles composed mainly of metal nitrideinto the fine matrix composed mainly of Si₃N₄, and smoothing of thedischarged surface after electric discharge machining is possible. Thus,even with such electric discharge machining, a less expensive conductivesilicon nitride composite sintered body which is excellent in mechanicalproperties can be provided.

1. A conductive silicon nitride composite sintered body comprisingsilicon nitride and metal nitride as its main components, saidcomponents each having an average grain size of 100 nm or less, saidsintered body including the metal nitride of 25% to 60% by volume, saidsintered body including, in an arbitrarily selected section thereof, aregion where the area ratio of the metal nitride is within a 2/3 powerratio of volume % of the metal nitride ±10% at a region of 2 μm wide and2 μm long.
 2. A conductive silicon nitride composite sintered bodycomprising silicon nitride and metal nitride as its main components,said components each having an average grain size of 100 nm or less,said sintered body including the metal nitride of 25% to 60% by volume,a discharged surface of said sintered body having the relative roughness(Ra) of 0.3 μm or less after electric discharge machining.
 3. Aconductive silicon nitride composite sintered body according to claim 1,wherein a metal of said metal nitride is at least one kind of elementselected from Groups IVa, Va and VIa of the Periodic Table.
 4. Aconductive silicon nitride composite sintered body according to claim 2,wherein a metal of said metal nitride is at least one kind of elementselected from Groups IVa, Va and VIa of the Periodic Table.
 5. Aconductive silicon nitride composite sintered body according to claim 3,wherein the metal of said metal nitride is Ti or Ta.
 6. A conductivesilicon nitride composite sintered body according to claim 4, whereinthe metal of said metal nitride is Ti or Ta.
 7. A process for producinga conductive silicon nitride composite sintered body, comprising thesteps of: preparing a silicon nitride powder, a powder of sintering aidsand a metal powder; grinding/mixing these powders by mixing for 24 hoursusing a planetary ball mill having an acceleration of 150 G, until theaverage particle size of the silicon nitride powder becomes 30 nm orless so as to obtain a mixed powder; molding the mixed powder to make amolded body; and sintering the molded body in a non-oxidative atmosphereat a sintering temperature of 1,100° C. to 1,300° C. to make a sinteredbody.
 8. A process for producing a conductive silicon nitride compositesintered body according to claim 7, wherein said crushing/mixing iscontinued until a peak of added metal in an X-ray diffraction patterndisappears.
 9. A process for producing a conductive silicon nitridecomposite sintered body according to claim 7, wherein said metal is atleast one kind of element selected from Groups IVa, Va and VIa.
 10. Aprocess for producing a conductive silicon nitride composite sinteredbody according to claim 9, wherein said metal is Ti or Ta.